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Of the Cerebellum.

3. The cerebellum seen from above presents a slanting flattened surface of the breadth of the os occipitis, which would be oval but that its central part is notched before and behind. The surface of the cerebellum is of grey nervous matter, and is disposed in concentric laminæ, separated by furrows from one to eight lines in depth. The under surface is divided into a hemispherical portion on either side, with concentric grey laminæ to each, and the shrunk central portion, separated by a shallow longitudinal furrow from either hemisphere, is transversely laminated: the central portion constitutes the upper and under vermiform processes. The hemispheres of the cerebellum are united anteriorly by a broad band of white matter, called the pons Varolii. Upon a section of either hemisphere of the cerebellum, the grey matter is seen to form a continuous layer of two lines in depth, spread over white nervous matter, which has the form of a stem dividing into branches which again divide and subdivide. The appearance is termed the arbor vitæ: towards its inner part a corpus fimbriatum is found of larger dimensions but of similar structure to that in the corpus olivare; it is open within and before. The cerebellum is united to the medulla oblongata by its inferior peduncles and by the corpora restiformia, to the brain by the upper peduncles or pillars of the valve of Vieussens. The middle peduncles are those which unite either hemisphere in the pons Varolii.

When the cerebellum has been hardened in alcohol, its white substance is found to be arranged in fibres upon the following plan.

One series of fibres is disposed in plates that are parallel to the surfaces of the lamina: these fibres are individually disposed in planes vertical or nearly so:

Of the Cerebellum.

223

they form media of communication between different parts of the grey surface of the cerebellum, joining together neighbouring laminæ, or laminæ more remote from each other, or finally laminæ belonging to separate lobes.

The central substance of the arbor vitæ is continuous with the peduncles of the cerebellum. The middle peduncle is the largest, is external, and spreads its fibres towards every part of the circumference of the cerebellum. The inferior peduncle is next in size and place, and distributes its fibres in greater proportion to the upper than to the under surface of the cerebellum. The upper peduncle, which is the smallest and innermost, enters the cerebellum covered by the preceding, in great part enters the corpus fimbriatum, and distributes its fibres more to the under than to the upper surface of the cerebellum. On tearing asunder the fibres of either peduncle they are found to be continuous, no doubt through intervals between the former series, with the grey matter of the surface.

The vermiform processes derive their white matter in part from the peduncles of the cerebellum, in part from the Vieussenian valve, which consists of a double series of white fibres, upon the upper surface of which a row of single laminæ is formed of identical structure to those of the upper vermiform process, with which they form a continuous series.

Thus the cerebellum is composed-of a folded layer of grey matter,―of white fibres which connect together the neighbouring and remote folds or laminæ of the same hemisphere, of white fibres which connect the grey matter of the opposite hemispheres,-of white fibres which

connect the grey matter of each hemisphere with the same side of the medulla oblongata,—and of white fibres which connect the grey matter of each hemisphere with the same side of the crus cerebri.

4. The cerebrum seen from above consists of two hemispherical masses, the surface of which is of a grey colour and divided by winding furrows of varying depth from a line to two inches, into the appearance of convolutions. The deepest furrow on the outside separates the anterior from the middle lobe; the deepest upon the inner surface separates the middle from the posterior lobe. The hemispheres cohere by means of a white `mass called the commissura magna cerebri: a section of the cerebrum shows that the grey matter forms a layer of the depth of about three lines containing white matter. When the cerebrum is hardened, the grey matter becomes fibrous, and tears vertically to the surface, and the white matter is found to be disposed on the same principle with that of the cerebellum. One series of fibres may be raised in plates which connect the grey matter of adjoining, as well as of distant convolutions of the same hemisphere. Another series collected principally in the corpus callosum and in the anterior commissure connects the convolutions of the opposite hemispheres; and each crus cerebri again spreads its fibres to the convolutions of the whole circumference of its hemisphere. The entire series of white fibres, which, associated with or forming the crus cerebri, diverge from the centre of the base of the brain towards its circumference, are composed, 1. of the anterior pyramid, the filaments of which pass through the intervals of the cross fibres of the pons Varolii, and are reinforced by others, which

Of the Ventricles in the Brain.

225

distinctly arise from the grey matter contained in these intervals: 2. of all the remaining parts of the medulla oblongata, excepting the corpus restiforme, and part of the posterior pyramid: 3. of the white matter of the valve of Vieussens, and of its pillars: 4. of filaments derived from the tubercula quadrigemina, from the optic thalamus, from the corpus striatum.

The ventricles of the brain are cavities in it, which in most instances result from the want of adhesion of opposite surfaces of adjoining parts, that have each their covering of pia mater: thus the fourth ventricle is the chamber interposed between the medulla oblongata, the cerebellum, and its peduncles; the third ventricle is the space between the receding surfaces of the thalami nervorum opticorum, and the crura cerebri; and the fifth ventricle is a hollow between the layers of the septum lucidum. In other instances ventricles are cavities wrought in particular organs of the nervous system, not left by the want of union of dissimilar parts in juxta-position. It may be remarked as a law in the formation of the brain, that whenever any part is very strikingly developed, a ventricle is formed in it. The ventricle in the optic tubercle of birds, the ventricle in the processus mammillaris in quadrupeds, the posterior horn of the lateral ventricle in man, a ventricle which exists in the pes hippocampi in man, and the ventricle in the large posterior part of the medulla spinalis in birds, may serve as illustrations of this remark.

The most serviceable impulse that has been given to the study of the anatomy of the brain of late years, we may attribute to the theoretical account given by Drs. Gall and Spurzheim: but the most valuable collection

226 Relation between the grey and white Matter.

of facts upon the same subject is contained in the essays of Reil. The happiest, and most original observation made by MM. Gall and Spurzheim consisted in pointing out the relation observed in the disposition of the grey and of the white matter in the brain. The examination of the hardened brain by the method of Reil further tends to show that the grey matter in the spinal chord, in the medulla oblongata, in the tubercles, in the corpora striata, and upon the surface of the cerebrum and cerebellum, constitutes the essential part of these organs, or that in which impressions are finally received or with which the phenomena of thought are perhaps immediately connected; while the rest of the mass appears composed of media of transmission only, calculated to convey to one part the influence derived from another.

The nerves in the human body are forty pairs of white chords, in form cylindrical or flattened, which extend from the spinal marrow and medulla oblongata to every organ in the body. The nerves while within the dura mater are either covered with an outer tunic of arachnoïd membrane, or are contained between the arachnoïd membrane and the pia mater. After perforating the dura mater the nerves are invested with a thick dense glistening tunic, which is called their cellular coat. Each nerve consists of many fibrils, that near the brain coalesce by oblique cross branches, and near the circumference of the body lie in parallel apposition only. Each fibril is invested by a delicate vascular membrane, which unites together all the fibrils that are contained within the outer tunic: this membrane is analogous to the pia mater; it forms the proper tunic of a nerve, and is termed the neurilema. Each fibril in a nerve consists